Bush is bound to be the center of attention — and attacks — when he joins his GOP rivals tonight for the first time at a debate in New Hampshire, an event that, in fair measure, will be all about Bush.

Just call it the George Gaffe Watch.

“What Bush could have gotten away with a month ago, he can’t get away with now,” says Democratic strategist David Doak. “People want to know if he’s up to the job — and he has to show them he is.”

The GOP front-runner has to answer all the chatter about whether he’s “Bush Lite,” sparked when he flunked a TV “gotcha” quiz on the leaders of four world hot spots. He got Taiwan right, but missed Pakistan, India and Chechnya.

Bush may have succeeded in changing the subject from his questionable foreign-policy prowess to a debate about tax cuts yesterday, when he released his $483 billion plan — which lowers taxes for Americans in every bracket, with some of the biggest cuts, percentage-wise, going to working poor families.

The “Bush Lite” talk calmed down when Bush gave a strong, Reaganesque foreign-policy speech and chose to make his Sunday TV talk-show debut on the toughest show — NBC’s “Meet the Press” with Tim Russert — and did just fine.

If he does well at the debate and doesn’t flub any more “gotcha” questions, the “Bush Lite” mess could be over. But that’s a big If. And there’s also The Smirk Factor.

The Smirk is actually an outgrowth of “Bush Lite” –fans see Bush’s grin as a charmingly wry and mischievous smile. But critics say it’s a smart-aleck smirk that shows Bush is really a Frat Boy who lacks the gravitas to be president.

So should Bush try to swallow The Smirk?

No, says Doak: “I’m a firm believer that these kinds of things that people tend to make a big deal about aren’t really that big a deal. And body language things are so hard to cure that usually the cure is worse.”

But Dem pollster Mark Mellman says The Smirk should go: “Those nonverbal cues can matter to people a lot more than anything you say. These things are not trivial.”

More important, Mellman adds: “Bush really has to do three things at once. He has to be folksy and appealing and, at the same time, sound serious and in command of the issues. And he needs to fend off attacks from his rivals. It’s not easy.”

Ah, yes, his rivals. Any one of the desperate also-rans, like Steve Forbes or Gary Bauer, may try to “win” the debate by knocking Bush off his perch with a zinger or a “gotcha” that becomes the sound bite of the evening, replayed endlessly.

Rising rival Sen. John McCain of Arizona poses a far more serious threat — even though he’s not likely to attack. For it’s McCain, the Vietnam POW hero, who is cited by many cite as the exemplar of gravitas. So how will Bush measure up when both men are on the same stage?

In fair measure, the Bush-McCain dynamic is the key to watch. How do they treat each other? Do they look like a team that could run together on the 2000 GOP ticket?

It’s a virtual cinch that someone will challenge Bush about whether his camp had any role in the nasty whispering campaign seeking to paint McCain as temperamentally unfit to be president because his POW years knocked him a bit wacky.

But that could be a high, inside fastball for Bush. He could hit a home run by blasting the smear campaign, declaring McCain a true American hero, and vowing to immediately fire anyone on his own staff who spreads such rumors.

That could be the sound bite — if Bush’s staff wasn’t involved in any of the whispering.

There’s also the quandary facing millionaire Steve Forbes, a distant third. He’s desperate to knock Bush down, but attacks in a multicandidate field get messy.

Voters get mad at attackers. So any votes Forbes takes away from Bush will likely ricochet and boost McCain into a stronger second — making Forbes even less relevant.